This invention relates to the measurement of electrical impedance, and in particular to the measurement of the loss and the very precise measurement of the capacitance of an unknown impedance where "loss" is used as a collective term to mean resistance, conductance, dissipation factor or any other term used to describe the real component of impedance. The technical literature is replete with numerous examples of impedance bridges of all kinds. These loss terms If are used equivalently and interchangeably below. Bridges have been in a state of continuous development and improvement for more than a century. Improvements have taken almost every conceivable form, in efforts to achieve higher accuracy, lower cost, better reliability, higher speed, wider range, etc. Some high performance bridges have been automated with the incorporation of microprocessors or related devices to allow these bridges not only to correct for various measurement errors, but to report their measurement results on sophisticated local displays or remotely via several different kinds of communication channels. Sufficient programming control is often provided to allow for sustained unattended operation.
In spite of the considerable attention given to impedance bridges in general, not all areas of bridge development have benefitted from new ideas, particularly in the application of microprocessors and digital signal processors (sometimes referred to below as DSP). Ratio transformer bridges have been used for high precision measurements of capacitance and loss. Several examples of ratio transformer bridges include those disclosed in U.S. Pat. No. 4,772,844. U.S. Pat. No. 4,772,844 describes a method for producing very accurate magnitude and phase relationships between two sinusoidal signals generated by two ratio transformers. Such signals are useful in a particular kind of ratio-transformer capacitance bridge where instead of balancing the resistive component of the unknown impedance against a reference resistor, it is instead, balanced against a reference capacitor driven by a 90.degree. phase-shifted signal.